1. Field of the Invention
The present invention relates to a method and apparatus for removing impurities such as suspended or dissolved substances from a liquid.
2. Description of the Prior Art
Ultra-pure water has recently come to be used in the manufacture of electronic appliance parts, medical appliance parts, and high-purity chemical products.
Such water, however, is only available at high cost. If a large amount of high-purity water is used, manufacturing costs are increased. In view of this, it has been proposed to remove impurities from used water so that it can be reused.
Even if the water is not reused, it must be purified to comply with water quality standards provided by the Water Pollution Preventive Act.
In addition, if untreated water such as underground water or river water can be purified to a satisfactory degree, the high product costs including purchasing of high-purity water can be reduced.
When dust or the like from the grinding of wafers such as silicon or gallium arsenide wafers in the semiconductor process becomes mixed in with water as an impurity, the dust is dispersed as colloidal particles.
In this case, the dust is suspended in the water and cannot be removed by normal filtering.
In reverse osmosis or ultrafiltration which is used in the manufacture of the ultra-pure water required in large quantities in the electronics industry and in particular in the manufacture of VLSIs, the substances most responsible for deposition on the filtering membrane or permeation membrane and degradation of filtering efficiency are believed to be silicic acid ions (HSiO.sub.3.sup.- or SiO.sub.3.sup.2-) dissolved in untreated water and the colloidal silicic acid suspended therein. The silicic acid colloidally suspended in the water is known to be converted upon reaction with water into orthosilicic acid, metasilicic acid, or metabisilicic acid to metatetra silicic acid. All of these substances can be dissolved in neutral water in only trace amounts, and are scarcely converted into silicic acid ions (e.g., SiO.sub.3.sup.2- or SiO.sub.4.sup.2-). it is difficult to remove colloidal silicic acid with a method using an anion exchange resin. In addition, that method can provide only a low adsorption rate of silicic acid ions and requires several passes of the untreated water through the ion exchange resin layer for the removal of ions. Still worse, other substances or ions such as Cl.sup.-, SO.sub.4.sup.2-, or CO.sub.3.sup.2- are preferentially removed and the pH of the water is increased to cause some troubles.
In order to remove solids from suspension, the suspended solids must be flocculated prior to filtration. This also applies to solutions containing a solute such as an organic substance.
Electrolysis is known as a first conventional method of flocculating impurities. In electrolysis, sulfuric acid or the like is added to a liquid in order to enhance the electrical conductivity of the liquid. Metal plates which cannot easily be corroded by electrolysis, e.g., plates obtained by coating lead with titanium, are used as electrodes.
A substance, when suspended in water, is normally charged. Therefore, the suspended substance is electrically neutralized and flocculated by the electrodes. A substance flocculated in this manner floats in fine bubbles of hydrogen or oxygen, and the floating flocculated substance can then be separated by filtration.
In the case of electrolysis, the electrolysis of a liquid is performed at atmospheric pressure using electrodes consisting of a soluble metal.
Japanese Patent Publication No. 59-5032 discloses a discontinuous method. In this method, a liquid contained in a pressure-resistant container is subjected to electrolysis under pressure. After performing electrolysis for a predetermined period of time, the pressure in the container is returned to normal pressure. Fine bubbles formed upon this pressure change attach to the floc and cause it to rise to the liquid surface. The floc can then be removed.
As a second conventional method of flocculating impurities, a large amount of a substance having a high degree of ionization such as a strong electrolyte, a strong acid, or a strong base is added to a liquid.
In that method, suspended substances are electrically neutralized, flocculated and then settled by the ion generated in electrolytic dissociation.
As a third conventional method of flocculating impurities, a base is added to a liquid after adding a water soluble metal salt. In this method, a metal hydroxide produced by neutralization between the metal salt and the base adsorbs impurities and precipitate them by flocculation. Therefore, the third method is effective for liquids containing as an impurity a soluble substance less charged such as organic substance as well as suspended substance.
In the conventional method wherein a liquid is electrolyzed at atmospheric pressure, a large amount of hydrogen gas is produced at the cathode. The amount of hydrogen gas produced is calculated by electrochemical equivalency to be 417.8 ml/hour per 1 A of current at 20.degree. C. and 1 atm. Hydrogen gas dissolves by 0.0182 ml per 1 ml of water at 20.degree. C. and 1 atm. In practice, however, most fine bubbles of hydrogen gas immediately join together and rise as large bubbles having a diameter of more than about 0.5 mm. For this reason, only a small proportion of the bubbles attach to impurities flocculated by the hydroxide and contribute to the flotation seperation. Assuming that a large tank having a sectional area of 0.05 m.sup.2 is used for 1L/min of liquid, about 95% of the total impurities in the liquid can be separated from the liquid surface, about 5% remains deposited on the tank bottom, and about 0.01% remains dissolved or suspended in the liquid unrecovered. Unless re-filtered through a filtering material having a mesh of about 0.5 .mu.m, pure water cannot be obtained and the consumption of filtering material becomes considerable.
With the second method, the added substance remains in the liquid at a high concentration as an additional impurity. Therefore, even if the suspended substance is completely separated, the treated water cannot be used for intended purposes without further treatments.
In the third method, the concentration of metal salt as a separation additive is low. However, further ions are produced in the liquid upon neutralization. Again, with the third method, the treated water cannot be used directly for intended purposes.
In order to allow the repeated use of pure water, ions produced in each purification process must be removed before reusing the water. Thus, a large amount of ion exchange resin must be used, and costs are increased and the advantage of reusing pure water is not realized.
A large quantity of ions also remains unfiltered in the first and second methods. In addition, the solid residue separated by flocculation contains large quantities of electrolytes or metal hydroxides. For this reason, melting or recrystallization of silicon wafers grinding dust cannot be performed, and the solid residue cannot be reused effectively, the residue being almost of no utility.
Furthermore, the known method of electroyzing a liquid under pressure is a discontinuous process as described above, and a continuous treatment method is preferable.